Models that operate in two or more systems using different frequency ranges, such as different GSM systems (Global System for Mobile telecommunications) have become increasingly common in mobile stations. The basic condition for the operation of the mobile station is that the radiation and reception properties of its antenna are satisfactory on the frequency bands of all the systems in use. Without any limit on size, it is relatively easy to make a high-quality multiband antenna. However, in mobile stations, especially mobile phones, the antenna must be small when it is placed inside the covering of the device for comfort of use. This increases the requirements of antenna design.
In practice, an antenna of sufficiently high quality that can be placed inside a small device can be most easily implemented as a planar structure: The antenna includes a radiating plane and a ground plane parallel with it. In order to facilitate matching, the radiating plane and the ground plane are usually connected to each other at a suitable point by a short-circuit conductor, whereby the resulting structure is of the PIFA (planar inverted F-antenna) type. The number of operating bands can be increased to two by dividing the radiating plane by means of a non-conductive slot into two branches of different lengths as viewed from the short-circuit point, in a way that the resonance frequencies of the antenna parts that correspond to the branches fall in the ranges of the desired frequency bands. However, in that case the adaptation of the antenna can become a problem. It is especially difficult to make the upper operating band of the antenna sufficiently wide when it is wanted to cover the bands used by two systems. One solution is to increase the number of antenna elements. An electromagnetically coupled, i.e. parasitic planar element is placed close to the primary radiating plane. The resonance frequency of the parasitic element is arranged e.g. close to the other resonance frequency of a two-band PIFA so that a uniform, relatively wide operating band is formed.
FIG. 1 shows such a known internal multiband antenna. In the figure there is the circuit board 101 of a radio device, which circuit board has a conductive upper surface. This conductive surface functions as the ground plane 110 of the planar antenna. At the one end of the circuit board there is a radiating plane 120 with a roughly rectangular outline, the plane being supported above the ground plane with a dielectric frame 150. From the edge of the radiating plane, close to a corner, starts the first short-circuit conductor 125 that connects the radiating plane to the ground plane, and the feed conductor 126 of the whole antenna. From the feed conductor, there is a ground isolated lead-through to the antenna port AP on the lower surface of the circuit board 101. The radiating plane 120 has been shaped by means of a slot 129 therein in a way that the plane is divided into two conductor branches of clearly different lengths as viewed from its short-circuit point, the PIFA thus being dual-band. The lower operating band is based on the first, longer conductor branch 121 and the upper operating band on another, shorter conductor branch 122. The antenna structure also includes a radiating parasitic element 130. This is a planar conductor piece on the same geometric plane with the radiating plane 120. The parasitic element is located adjacent to the radiating plane on its long side next to the first portion of the first conductor branch mentioned above. One end of the parasitic element is connected to the ground with the second short-circuit conductor 135, which is relatively close to the feed conductor 126. In that case, the electromagnetic coupling between the parasitic element 130 and the radiating plane 120 is obtained to be strong enough to make the parasitic element function as a radiator. Together with the surrounding structure, the parasitic element forms a resonator which has a natural frequency on the band of the PCS1900 system (Personal Communication Service), for example. If the natural frequencies of the PIFA have then been arranged on the bands of the GSM900 and GSM1800 systems, for example, the result is an antenna that operates in three systems.
The structure according to FIG. 1 has the drawback that the parasitic element is relatively sensitive to external conductive materials. Therefore, the mobile phone user's hand can significantly impair the band properties of the antenna. In addition, the matching of the antenna on the lower operating band leaves room for improvement.